This application claims the benefit of U.S. Provisional Application No. 60/258,396 filed Dec. 27, 2000.
This invention relates generally to inspection tools, and more particularly to inspection tools for examining nuclear reactor jet pump assembly welds.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. The slip joint between the jet pump inlet mixer and the jet pump diffuser collar has about 0.015 inch diametral operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump.
The inlet mixer and the diffuser, due to their large size, are formed by welding a plurality of cylindrical sections together. Specifically, respective ends of adjacent cylindrical sections are joined with a circumferential weld. During operation of the reactor, the circumferential weld joints may experience intergranular stress corrosion cracking (IGSCC) and irradiation assisted stress corrosion cracking (IASCC) in weld heat affected zones which can diminish the structural integrity of the jet pump.
It is important to examine the welds of the jet pump inlet mixer and diffuser periodically to determine whether any cracking has occurred. While examinations in the annulus, or region between the shroud and the pressure vessel wall, can be performed, these examinations are likely to be only partial inspections due to access limitations in the annular region of the reactor. Any examination in the annulus is hampered by the numerous components and further constrained by any shroud repair hardware present in the annulus. Thus, it is highly advantageous to perform the examination of the jet pump welds from the inside of the jet pump inlet mixer and jet pump diffuser.
It would be desirable to provide an inspection tool that is capable of performing ultrasonic and/or eddy current examinations of jet pump welds from inside the jet pump inlet mixer and diffuser in a nuclear reactor. It would also be desirable to provide an inspection tool that is remotely operable and capable of providing position information relative to fixed items within the reactor.
In an exemplary embodiment, an inspection apparatus for inspecting welds in a nuclear reactor jet pump fits inside the jet pump and inspects the circumferential welds in the jet pump inlet mixer and jet pump diffuser. The inspection apparatus includes a frame structure configured to attach to a top flange of the reactor pressure vessel (RPV) of the nuclear reactor. A first motor is movably coupled to the frame structure and operatively coupled to a flexible drive cable. A tool head is coupled to the flexible cable. The tool head includes a first portion coupled to a second portion by a first flexible U-joint, and a probe subassembly coupled to the second portion by a second flexible U-joint.
The frame structure includes an elongate frame member, an attachment frame member extending from one end portion of the elongate frame member, and a support wheel coupled to the other end portion of the elongate frame member. The attachment frame member is configured to attach to the top flange of the reactor pressure vessel when the reactor pressure vessel head is removed. When the inspection apparatus is installed in the RPV, the support wheel engages the side wall of the RPV. An elongate track is coupled to the elongate frame member and extends substantially the length of the frame member.
The probe subassembly includes a probe housing and a plurality of probe arms pivotably coupled to the housing at one end of each probe arm. Each probe arm includes a sensor coupled to the opposing end of the probe arm. The probe arms are pivotably movable between a first position where the probe arms are parallel to a longitudinal axis of the probe subassembly, and a second position where the probe arms are at an angle to the longitudinal axis of the probe subassembly. In the second position, the sensors contact the inner surface of the jet pump to inspect the welds. The attached sensors are ultrasonic transducer probes and/or eddy current transducer probes. The first motor rotates the drive cable around the longitudinal axis of the drive cable. This axial rotation of the drive cable causes the probe subassembly to rotate around the longitudinal axis of the tool head and moves the sensors circumferentially around the inside wall of the jet pump.
A second motor is mounted on a trolley which is movably coupled to the elongate track. The second motor moves the trolley along the track. The first motor is also mounted on the trolley. The movement of the trolley along the track moves the flexible cable and the attached tool head so as to position the tool head at various vertical heights within the reactor pressure vessel.
An insertion subassembly couples to the jet pump inlet. The insertion subassembly is sized to receive the tool head and the connected flexible drive cable and guide the tool head and flexible drive cable into the jet pump through the jet pump inlet. The insertion subassembly includes an elongate tube portion, a location cone attached to one end of the tube portion, and an attachment clamp attached to the other end of the tube portion. The attachment clamp is configured to clamp to the jet pump suction inlet. Particularly, the attachment clamp includes a plate coupled to the tube portion. The plate includes a notch sized to receive the side wall of the jet pump suction inlet. The attachment clamp further includes an engagement arm pivotably coupled to the plate and a ratchet assembly coupled to the engagement arm. The engagement arm is movable into engagement with the jet pump inlet by tightening the ratchet assembly.
The above described inspection apparatus performs ultrasonic and/or eddy current examinations of jet pump welds from inside the jet pump inlet mixer and diffuser in a nuclear reactor without having to disassemble the jet pump. Also the inspection tool is remotely operable and provides position information relative to fixed items within the reactor.